The present invention generally relates to semiconductor devices and more particularly to a fabrication process for a semiconductor device that has an insulating film of silicon dioxide.
Various semiconductor devices utilize silicon oxide for the insulating film. For example, the silicon oxide film is used for the gate insulating film of MOS transistors or for the dielectric layer of storage capacitors of memory cells. Such silicon oxide films may be formed by the CVD process or thermal annealing process on a silicon substrate made in the oxidizing atmosphere.
With the requirement of reduced size and increased integration density of semiconductor devices, the thickness of the silicon oxide film is reduced progressively, and according to the scaling law. This in turn invites the concentration of the electric field in the thin oxide film that eventually leads to the breakdown of the film caused by the current flowing therethrough.
FIGS. 1(A)-1(C) show the band diagram of a conventional MOS diode under a biased state.
Referring to the structure shown in FIG. 1(A), the MOS diode comprises a p-type silicon substrate 11, a silicon oxide film 12 grown on the substrate 11, and a doped polysilicon electrode 13 grown further on the silicon oxide film 12. Upon the negative and positive biasing applied by a voltage source V across the silicon substrate 11 and the polysilicon electrode 13, the conduction band Ec and the valence band Ev of the silicon substrate 11 curve variously at the interface between the silicon substrate 11 and the silicon oxide film 12. Thereby, there appear various states of the band structure, such as the accumulation state or the inversion state, as is well known in the art.
FIG. 1(A) shows the accumulation state in which a negative voltage is applied to the electrode 13 of the MOS diode. In this state, a large electric field is induced in the silicon oxide film 12 as a result of the negative biasing. It will be easily understood that the intensity of the electric field in the silicon oxide film 12, represented by the gradient of the conduction and valence bands, increases with decreasing thickness of the film 12.
When the quality of the silicon oxide film 12 is ideal, no injection of electrons occurs from the polysilicon electrode 13 to the silicon oxide film 12, unless the applied bias voltage becomes extremely large and the effective barrier width H1 shown in FIG. 1(A) decreases to a size that allows the tunneling of electrons. However, an actual silicon oxide film 12 generally includes various defects therein and it is inevitable that small amount of electrons is injected into the film 12 through the energy levels pertinent to these defects.
Once the electrons are injected, they experience acceleration due to the large electric field established in the film 12. This acceleration of electrons is particularly conspicuous in the devices that have a thin silicon oxide film. Thereby, the electrons cause impact ionization upon collision with the atoms forming the silicon dioxide, and such impact ionization induces formation of the hole-electron pairs. Generally, the electrons thus formed escape to the silicon substrate relatively easily due to their high mobility, while there is a tendency that the holes thus formed are trapped in the silicon oxide film 12 because of their low mobility. This effect is not significant at the beginning of device operation, as the leakage of current through the silicon oxide film 12 is extremely small at the beginning. However, the continuous use of the device inevitably invites accumulation of the holes in the silicon oxide film 12, and such holes trapped in the film 12 tend to lower the energy level of the conduction band as well as the valence band thereof. Thereby, the conduction band Ec and the valence band Ev of the silicon oxide film 12 are curved downward as shown in FIG. 1(B), and the effective barrier width H2 is inevitably decreased.
With further accumulation of the holes, the effective barrier width H2 of the silicon oxide film 12 decreases further, and the probability of electrons tunneling through the silicon oxide film 12 increases. There, the electrons at the Fermi level E.sub.F of the gate 13 reach the conduction band Ec of the silicon oxide film 12 and flow to the silicon substrate 11 with finite probability. Each of these electrons experiences acceleration produced by the large electric field in the silicon oxide film 12 and induces new hole-electron pair formation by the impact ionization. Thereby, the concentration of the holes accumulated in the silicon oxide film 12 increases with such acceleration, and the MOS diode ultimately reaches a state shown in FIG. 1(C) wherein the silicon oxide film 12 no longer works as effective barrier. In this state, the MOS diode is broken down. It should be noted that the transition from the state of FIG. 1(B) to FIG. 1(C) occurs in an extremely short time period. This accumulation of the holes in the silicon oxide film is believed to be one of the major causes that decreases the lifetime of the MOS diodes or MOS transistors. It should be noted that the above argument applies also to the case of other semiconductor devices which use a insulating silicon oxide film, such as memory cells.
Conventionally, it has been known that the addition of chlorine or fluorine ions at the time of formation of the silicon oxide film contributes to the improvement of the quality of the obtained oxide film. Further, it is reported in the Japanese Laid-open Patent Application 53-121466 that the ion implantation of C1 ions into the silicon oxide film is effective for removing the impurities from the oxide film and for improving the quality of the film. However, as will be described later, it was found that the incorporation of C1 ions in the silicon oxide film causes a wide scattering of the breakdown voltage. Such a process is unreliable and cannot be accepted for the practical fabrication process of semiconductor devices.